"Microwave" is the term generally used to describe the portion of the electromagnetic spectrum that has wavelengths (.lambda.) between the far infrared and the radio frequency; i.e. between about one millimeter and about 30 centimeters, with corresponding frequencies (.nu.) in the range from about 1 to 100 gigahertz (GHz). Microwave radiation has a number of useful purposes, including spectroscopy, communication, navigation, and medicine, but one of the most common uses is as a heating technique, particularly for food; i.e. the almost ubiquitous "microwave oven."
Because heating is such an integral step in so many chemical processes, the potential for using microwave as a heating source for chemical processes has been recognized for some time, and a number of devices and methods have been developed for microwave assisted chemistry, including analytical chemistry. Analytical chemistry can be roughly defined as those methods used to identify one or more of the components (compounds, elements and mixtures) in a sample of material, as well as the determination of the relative quantity of each component in such a sample. As is well known to those of ordinary skill in the chemical arts, analytical chemistry is a major area or interest from a practical standpoint.
The process of identifying she components is generally referred to as "qualitative" analysis, and the determination of the amounts of various components is generally referred to as "quantitative" analysis. Examples of qualitative and quantitative analyses are numerous. Specific ones include (but are certainly not limited to) measurement of pollutants or other components of gases; identification of components in blood or other tissue for medical purposes; the production, control, and safety of food products; the manufacture of major industrial materials such as acids, organic chemicals, steel and the like; and the analysis of soil and other related materials for agricultural and related purposes. Additionally, such quantitative and qualitative analyses is often foundational to fundamental research activity in the basic sciences such as chemistry, biology, and biochemistry.
In many cases, quantitative and qualitative analyses are proceeded by preliminary steps that are required to give the analytical data the appropriate accuracy and significance. Typical steps include gathering an appropriate sample of the material to be analyzed, turning that into an appropriate mixture or composition for analytical purposes, and often drying the sample or otherwise determining its moisture content. For example, "oven drying" is a classical method for drying a sample (and thus determining its moisture content) based on the change in weight during drying. As is known to those familiar with chemical processes, oven drying is generally time consuming and in many cases must be followed by an appropriate cooling period, because a hot sample (or even a warm one) can cause problems during the weighing process. For example, a hot sample tends to set up convection air currents that disturb an otherwise sensitive balance.
Analytical chemistry also often requires performing measurements on solutions rather than on raw materials. Thus, the particular composition to be identified or measured (the "analyte") must often be converted into a soluble form. Such treatment usually requires powerful reagents such as concentrated mineral acids and strenuous treatment including relatively high temperatures. Microwave radiation can be used to heat such solutions, particularly when they are aqueous or aqueous based (e.g. mineral acids, such as hydrochloric, nitric and sulfuric), but offers the disadvantages noted above.
Similarly, analysis of an elemental composition or organic sample generally requires a relatively severe treatment to convert compounds into elemental forms that are either convenient or even necessary in many common analytical techniques. Such treatments usually represent oxidation of the sample and thus include conversion of carbon to carbon dioxide and hydrogen to water or water vapor. Some of the oxidation procedures that use liquid oxidizing agents such as the mineral acids are referred to as "wet ashing," "wet-oxidation," or "digestion."
As alternative, dry ashing or dry oxidation usually refers to the processes in which the organic compound is ignited in air or oxygen. In each case, the requirement for high temperatures makes microwave processes attractive apart from the noted disadvantages.
As an another chemical analysis technique where heat can be useful, many compounds are separated by the use of extraction procedures; i.e. taking advantage of the distribution of a solute between two immissible phases. Because extraction is fundamentally an equilibrium process, the application of heat can be particularly useful, and indeed the use of microwaves for this purpose has been suggested by Pare et al. in processes described in U.S. Pat. No. 5,002,784 among others.
Other uses of heat in chemical processes include simple evaporation of liquids for the straightforward purpose of decreasing the volume of a solution without loss of a nonvolatile solute. As noted above, drying or igniting a sample to constant weight also requires heat, and thus microwave processes form an attractive alternative to the classical use of burners, hot plates, and convection ovens.
For several generations of chemists, heating has typically been done with the classic bunsen burner, or more recently heated plates ("hot plates"). Nevertheless, the use of microwave energy is entirely appropriate, if all other factors are likewise conducive to use of the microwaves. Because water and a number of organic compounds are good absorbers of microwave energy, the use of microwaves provides an attractive alternative, at least in concept, to such traditional heating methods.
Accordingly, there are a number of commercially available microwave devices that are designed for laboratory use.
When microwave devices are used for chemical reactions, a common technique for maximizing their efficiency is to run a plurality of reactions in separate containers ("vessels") at the same time in a single, relatively large resonator. The containers are typically made of a microwave transparent material such as an appropriate plastic or ceramic. Generally a plurality of two or more containers, and sometimes as many as fifty, are placed in the cavity of a laboratory microwave oven and then radiated with the microwaves. In a typical circumstance, one of the vessels is monitored for pressure, temperature, color change, or some other parameter that measures or indicates the progress of the reaction in that single vessel. The remaining unmonitored vessels are considered to have behaved identically to the monitored vessel. This is, however, only a best estimate, as is recognized by those of ordinary skill in this art. Accordingly, the methods carried out by such typical apparatus offer less-than ideal results in many circumstances.
Processes for heating chemical reactions also have other limitations, however, a number of which arise from the volatility of many compounds, particularly organic compounds, at higher temperatures. As well known to chemists, water's boiling point of 100.degree. C. is relatively high for such a small molecule and results from its propensity for hydrogen-bonding. Many larger organic molecules have lower boiling points, meaning that they become volatile at lower temperatures. Because gas volumes expand rapidly with temperature (pV=nRT), analytical reactions that produce gases must be either carefully vented or carried out in pressure-resistant or pressure-controlled equipment.
Alternatively, if a particular analysis requires heating an otherwise volatile material beyond its atmospheric boiling point while preventing its evaporation, the reaction must be carried out at elevated pressures, and will accordingly require pressure vessels and associated operating parameters and safety equipment.
For example, analysis reactions such as digestion in which the oxidizing agent is concentrated (70%) nitric acid (HNO.sub.3 ; boiling point 120.5.degree. C.) must either be limited to temperatures below 120.5.degree. C. at atmospheric pressure, or must be carried out at elevated pressures in order for the temperature to be raised above 120.5.degree. C.
Accordingly, the need exists for a technique for heating and driving chemical reactions that can be carried out at elevated temperatures and atmospheric pressure, and that can accordingly incorporate reagents that would otherwise require gas and pressure control under most circumstances.